Well, some one beat me to the punch about Automatic feeders with a discussion but I'll post what I had started anyway.
When I started doing aquaponics I had no idea how much difficulty I would have finding an appropriate automatic fish feeder. And finding an automatic fish feeder was rather important since I have to be able to leave the system to the attention of neighbors fairly often when I need to travel. (I don't want to ask a neighbor to come over and measure out feed three times a day for multiple tanks.)
First we tried a solar powered automatic game feeder. Unfortunately those feeders fling the feed out in all directions so I had to sort out something to catch the feed and drop it down into the fish tank. And then once we sorted that out, we had some nocturnal visitor that would mess the thing all up and leave it hanging all crooked.
So, I tried a few demand feeders but the catfish didn't seem interested in pulling or bumping anything in order to get their food (but I expect the raccoon did.)
Next we tried a pond feeder meant for flinging feed out into farm or game ponds. Well that thing would fling the feed so hard that it would pulverize against whatever top I tried to use to get it to stop and drop down into the fish tank. So we modified it to make it fling less hard but it still never really worked well and the feed wouldn't all move down into the shoot and the timer wasn't adjustable enough for the situation and would either under feed or over feed.
I looked at the professional fish feeder systems used in aquaculture but the prices were high and some of the same problems would still apply for a smaller aquaponics system.
There are some koi feeders out there that might work but no one really gave any glowing recommendations of them to me so I didn't want to spend so much on a feeder that didn't hold much reserve of feed.
Next option was to modify an automatic feeder like used for cats/dogs. Well we tried a couple but didn't have satisfactory results from any of the less costly ones.
I finally splurged and spent the money to get an Ergo Automatic Pet Feeder.
The feeder works well to deliver precise amounts of feed and the timer is very adjustable. However the feeder is not really designed for outdoor use (they do make a pond feeder version but it is simply a cover over it for weather proofing. The first winter using the feeders, I discovered that they are not rat proof. I had damage to the feeders from rats chewing on them. Solution was to protect with hardware cloth which makes cleaning the feeder much more difficult and I've not figured out a good way to protect the feed shoot from rats trying to climb into the feeder through the auger tube. (In this process I learned that the Rat Zapper Ultra is effective at killing rats.)
So it works but I highly recommend some modifications before using it in an outdoor or greenhouse situation.